Blind rivets (i.e. rivets which can be installed by access to one side only of the workpiece) are well known. Commonly a blind rivet comprises a tubular shell having an elongated shank with a preformed radially enlarged shell head at one end (the head end), in combination with a stem extending through the tubular shell and having a radially enlarged stem head at one end thereof (the head end) adjacent the other end (the tail end) of the shell shank. The other end portion of the stem protrudes from the head end of the shell. The shell shank is inserted through aligned apertures in the workpiece comprising the members to be riveted together so that the shell head abuts the near face of the workpiece and the tail end portion of the shell shank protrudes beyond the remote face (the blind face) of the workpiece. An increasing pulling force is then applied to the protruding portion of the stem relative to the shell, the reaction force being supported by the shell head, so that the stem head deforms the tail end portion of the shell shank radially outwards and axially towards the shell head, to form a blind head which abuts the blind face of the workpiece. The workpiece members are thus clamped together between the shell""s preformed head and its blind head. Usually the stem is then broken off flush with, or slightly inside, the head of the shell, at a breakneck preformed at the appropriate position along the stem. The breakneck breaking load is at a tension load which is greater than the load needed to completely form the blind head.
Such blind rivets and the method of using them are well known.
Blind rivets which provide a high level of static and dynamic joint strength need to develop a high retained compressive force on the workpiece, between the preformed and blind heads, and to have a relatively large preformed head and also a blind side head which has a relatively large diameter in contact with the blind face of the workpiece, i.e. a relatively large blind side footprint. An example of such a blind rivet is described in GB 2 151 738 A, and is widely available under the registered trademark HEMLOK.
One problem with such high joint-strength rivets in the past is that they have been restricted in the amount of joint gap closure they can provide, i.e. the amount of gap initially present between the members to be joined, which the rivet can successfully close up during installation in the members, is limited.
The present invention aims to overcome this problem, and aims to provide a blind rivet which develops a large blind-side head footprint, an enhanced sheet gapclosing ability and also produces a large compressive force on the completed joint.
GB 613882 discloses a blind rivet having a shell without a preformed head, and a method of riveting involving applying axial compression to the shell to form both the blind and near side heads. However the rivet is such that formation of the near side head is completed before formation of the blind side head begins. Furthermore the rivet comprises only a tubular shell without a stem, the placing tool being provided with a reusable mandrel which is removed from the rivet shell after the latter has been completely deformed.
GB 511588 (Chobert), a divisional of GB 511,531, describes a tubular riveting system for securing workpieces together. This earlier method employs a pull-through mandrel having an enlarged head. The riveting tool incorporates an inner sleeve around the mandrel, the sleeve having a smaller diameter than the hole in the workpieces and smaller diameter than the undeformed rivet. The workpieces are thus constrained against the force of the mandrel by the outer part of the tool. However, this prior art relates to pull-through riveting and is not directly applicable to breakstem riveting. Furthermore, the dimensions of the riveting tool do not allow space for a head to form on the rivet unless a countersink is provided in the workpiece nearest the tool.